MEXICO CITY, July 18 (Reuters) - Mexico’s Senate gave general approval early on Friday to a key bill to regulate the opening of the oil and gas industries to private investment, the centerpiece of President Enrique Pena Nieto’s economic reform agenda.

The Senate voted 90-28 to approve the general outline of the hydrocarbons law, which details the rules for private contracts and fines, but lawmakers reserved dozens of articles for further debate before a final vote.

The bill fleshes out a historic overhaul of the state-run energy sector approved late last year and it will pass to the lower house from the senate.

Following a decade of falling oil and gas output, Pena Nieto pushed through a reform in December 2013 to end state oil giant Pemex’s 75-year monopoly and allow for production and exploration by private companies.

The reform is the central plank of Pena Nieto’s plan to boost growth in Latin America’s No. 2 economy, which has lagged behind more dynamic emerging markets.

Senators are set to vote on three more energy bills, including one that opens up the electricity market to private companies. (Reporting by Michael O‘Boyle; Editing by Clarence Fernandez and Muralikumar Anantharaman)